U.S. Pat. No. 3,932,681 discusses the general practice of applying coating compositions to selected portions only of vitreous (glass), glass-ceramic, and ceramic articles through the use of masking techniques. Hence, those portions of the articles which are not to receive a coating are covered with a masking material which is inert both to the vitreous, glass-ceramic, or ceramic substrate and to the coating composition to be applied, and which is readily removable from the substrate after the desired functional coating has been applied thereto. Inasmuch as the coating compositions are frequently applied while at quite elevated temperatures, e.g., greater than 500.degree. C. and, occasionally, higher than 700.degree. C., the masking material must demonstrate a softening point or melting point greater than the temperature to be encountered during the coating process. Accordingly, the masking materials may be required to maintain their particulate nature at temperatures in excess of 1000.degree. C.
As explained by the patentees, the mask must be impermeable and capable of being readily removed from essentially non-porous vitreous, glass-ceramic, or ceramic surfaces even after being fired to temperatures of 700.degree. C. and higher. The masking compositions described in that patent consisted of about 40-90% by weight of bentonite clay and about 10-60% by weight of refractory metal oxide release agent, the latter being preferably selected from the group of silica, titania, iron oxide, tin oxide, calcium oxide, 96% silica, and alundum cement. Both constituents are utilized in a very finely-divided state, viz., the particles pass through a 100 mesh United States Standard Sieve (149 microns).
Bentonite is a clay having a volcanic origin wherein the major constituent is the mineral montmorillonite. Although its chemical composition is similar to many commercially-available clays, the physical constitution thereof is different therefrom in containing more colloidal matter.
The bentonite clay and release agent were suspended in a water-containing vehicle selected from the group of water and mixtures of water with lower alkanols, viz., alkanols of 1-4 carbon atoms. The preferred vehicle consisted of about 20-50% by volume of methanol. The resultant slurry was applied to the substrate via conventional means, e.g., brushing, dipping, rolling, or spraying, and the vehicle was subsequently removed by heating the substrate surface to a temperature above the boiling point of the vehicle. Typically, the slurries consisted of 10-35% by weight solids and 65-90% by weight liquid vehicle.
The masking composition was readily removed from the substrate surface after the application of the desired functional conductive, decorative, protective, or reflective coating to the vitreous, glass-ceramic, or ceramic ware. Such second coating could be, for example, an electrically-conductive metal oxide, a decorative glass enamel, a metal oxide stain, or any other similarly functional coating. The removal of the masking was readily accomplished through rinsing in water with occasional light hand rubbing even after the masking composition had been fired at temperatures up to about 700.degree. C. The inclusion of the refractory metal oxide release agent, however, was demanded to insure ready removal from the ware surface.
A more detailed discussion of compositions operable as masking materials for vitreous, glass-ceramic, and ceramic ware and of the method for applying and removing such materials is provided in said U.S. Pat. No. 3,932,681, and specific reference is directed to that disclosure for such discussion.
It has been appreciated that in certain applications it would be useful to employ a masking coating which could print a design, a pattern, or other representation onto the surface of vitreous, glass-ceramic, or ceramic ware while concurrently providing the function of an impermeable masking covering. Such a coating could be utilized, for example, for date coding, for affixing a trademark, or for applying a decorative design, where the surface beneath the masking formulation is an area not permanently covered with a functional conductive, decorative, protective, or reflective coating. The bulk of the masking coating, in like manner to that described in U.S. Pat. No. 3,932,681, would be readily removable from the ware surface leaving the ware surface bare.